Experiment 8



Gravimetric Analysis of a Chloride Salt

Objective

To illustrate typical techniques used in gravimetric analysis by quantitatively determining the amount of chloride in an unknown.

Materials and Chemicals

Balance ringstand, ring, and wire gauze

250-mL beaker (6) stirring rods (3)

Bunsen burner Rubber policeman (3)

Funnels (3) sharkskin filter paper (3)

Funnel support watch glasses (3)

Plastic wash bottle weighing paper

Graduated cylinders (2), 10 and 100 mL unknown chloride sample

0.5 M AgNO₃ acetone

6 M HNO₃ distilled water

Discussion

Quantitative analysis is that aspect of analytical chemistry concerned with determining how much of one or more constituents is present in a particular sample of material. Information such as percentage composition is essential to establishing formulas for compounds. Two common quantitative methods used in analytical chemistry are gravimetric and volumetric analysis. Gravimetric analysis derives its name from the fact that the constituent being determined can be isolated in some weighable form. Volumetric analysis, on the other hand, derives its name from the fact that the method used to determine the amount of a constituent involves measuring the volume of a reagent. Usually, gravimetric analyses involve the following steps:

1. Drying and then accurately weighing representative samples of the material to be analyzed.

2. Dissolving the samples.

3. Precipitating the constituent in the form of a substance of unknown composition by adding a suitable reagent.

4. Isolating the precipitate by filtration.

5. Washing the precipitate to free it of contaminants.

6. Drying the precipitate to a constant mass ( to obtain an analytically weighable form of unknown composition).

7. Calculating the percentage of the desired constituent from the masses of the sample and precipitate.

Although the techniques of gravimetric analysis are applicable to a large variety of substances, we have chosen to illustrate them with an analysis that incorporates a number of other techniques as well. Chloride ion may be quantitatively precipitated from solution by the addition of silver ion according to the following ionic equation:

[pic]

Silver chloride is quite insoluble (only about 0.0001 g of AgCl dissolves in 100 mL of H₂O at 20⁰C); hence, the addition of silver nitrate solution to an aqueous solution containing chloride ion precipitates AgCl quantitatively. The precipitate can be collected on a filter paper, fried, and weighed. From the mass of the AgCl obtained, the amount of chloride in the original sample can then be calculated.

This experiment also illustrates the concept of Stoichiometry. Stoichiometry is the determination of the proportions in which chemical elements combine and the mass relations in any chemical reaction. In this experiment stiochiometry means specifically the mole ratio of the substances entering into and resulting from the combination of Ag⁺ and Cl⁻. In the reaction of Ag⁺ and Cl⁻ in Equation [1], it can be seen that 1 mol of chloride ions reacts with 1 mol of silver ions to produce 1 mol of silver chloride. Thus,

[pic]

The number 0.2473 g is called a gravimetric factor. It converts grams of AgCl into grams of Cl. Gravimetric factors are used repeatedly in analytical chemistry and are tabulated in handbooks. The percentage of Cl in the sample can be calculated according to the following formula:

[pic]

[pic]

Procedure

On a piece of weighing paper, weigh to the nearest 0.0001 g about 0.2 to 0.4 g of your unknown

sample. Transfer the sample quantitatively to a clean 250-mL beaker (do not weigh the beaker) and label the beaker #1 with a pencil. Record the sample weight. Add 150 mL of distilled water and 1 mL of 6 MHNO₃ to the beaker. Repeat with sample numbers 2 and 3 and label the beakers #2 and #3, respectively. Using different glass rods for each solution, stir until all of the sample has dissolved. Leave the stirring rods in the beakers. Do not place them on the desktop.

While stirring one of the solutions, add to it about 20 mL of 0.5 M AgNO₃ solution. Place a watch glass over the beaker. Warm the solution gently with your Bunsen burner (or hot plate) and keep it warm for 5 to 10 min. Do not boil the solution. Low setting!!

Obtain a filter paper (three of these will be needed) and weigh it accurately. (Be certain that you weigh the paper after it has been folded and torn, not before.) Fold the paper as illustrated in Figure 8.1 and fit it into a glass funnel. Be certain that you open the filter paper in the funnel so that one side has three pieces and one side has one piece of paper against the funnel-not two pieces on each side. Why? Your instructor will also demonstrate this for you. Wet the paper with distilled water to hold it in place in the funnel. Completely and quantitatively transfer the precipitate and all the warm solution from the beaker onto the filter, using a rubber policeman (your laboratory instructor will show you how to use a rubber policeman) and a wash bottle to wash out the last traces of precipitate. The level of solution in the filter funnel should always be below the top edge of the filter paper. Wash the precipitate on the filter paper with two or three 5-mL portions of water from the wash bottle. Finally, pour three 5-mL portions of acetone through the filter. (Caution: Acetone is highly flammable! Keep it away from open flames.) Remove the filter paper, place it on a numbered watch glass, and store it in your locker until the next period.

Repeat the above processes with your other two samples, being sure that you have numbered you watch glasses so that you can identify the samples. The precipitated AgCl must be kept out of bright light, because it is photosensitive and slowly decomposes in the presence of light as follows:

[pic]

In this equation hv is a symbol for electromagnetic radiation; here it represents radiation in the visible and ultraviolet regions of the spectrum. This is the reaction used by Corning to make photosensitive sunglasses. In the next period, when the AgCl is thoroughly dry, weigh the filter papers plus AgCl and calculate the mass of AgCl. From these data calculate the percentage of chloride in your original sample.

Accuracy: correctness of a measurement, closeness to the true result.

Precision: internal consistency among one’s own results, that is, reproducibility.

Error: difference between the true result and the determined result.

Determinate errors: errors in method of performance that can be discovered and eliminated.

Indeterminate errors: random errors, which are incapable of discovery but which can be treated by statistics.

Questions

1. What is the fundamental difference between gravimetric and volumetric analysis?

2. Why should silver chloride be protected form light? Will your result be high or low if you don’t protect your silver chloride from light?

3. Can you eliminate indeterminate errors from your experiment?

4. Suggest another compound that could be used instead of AgCl. Justify why it would work.

|Gravimetric Analysis of a Chloride Salt |

|Report Sheet |Trial 1 |Trial 2 |Trial 3 |

|Mass of Sample |  |  |  |

|Mass of filter paper + AgCl |  |  |  |

|Mass of filter paper |  |  |  |

|Mass of AgCl |  |  |  |

|Mass of Cl in original sample (show calculations) |  |  |  |

|Percent chloride in original sample (show calculations) |  |  |  |

|Average percent chloride (show calculations) |  |

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